1. Field of the Invention
This invention relates to plasma processing apparatus.
2. Description of the Related Art
Plasma processing apparatus is used extensively in the fabrication of semiconductor devices and in many other processes in which the deposition of coatings or the etching of surfaces is required. Particularly when these processes are being carried out on a production scale, uniformity between batches becomes extremely important.
Generally such plasmas are generated in a vacuum chamber and it is preferred that the means for generating the plasma is either located outside the chamber or within a protective wall, in order to reduce the disturbance to the plasma potential when radio frequency power is applied.
Thus there have been suggestions of surrounding the plasma chamber by a single coil as in U.S. Pat. No. 4,844,775 and a number of Parties have suggested placing a single spiral coil against one end of a processing chamber. An example of this is EP-A-0379828. Usually a dielectric window is provided adjacent the coil. U.S. Pat. No. 5,309,063 describes a variation on this in which the single coil in re-entrant into the plasma chamber within a dielectric cup and proposals have been made in U.S. Pat. Nos. 52,801,154 and 5,397,962 for a plurality of spaced antennae located so as to allow the creation of rotating electro-magnetic fields. U.S. Pat. No. 5,468,296 discloses a plasma generating apparatus formed as a series of interconnected concentric rings and connected to a single power source. EP-A-489407 provides a stack of single turn antennae which are positioned one above the other at the same radial distances from the axis of a plasma generating reactor. U.S. Pat. No. 5,309,063 discloses a plasma generator wherein a single spiral coil is set into a cup-shaped window set into the centre of the top wall of the processing chamber of the device. WO-A-96 18208 illustrates a plasma processor having a planar coil located adjacent to individually supported dielectric windows of a processing chamber.
The aforementioned proposals suffer to one degree or another from lack of uniformity in the plasma, for example in the vicinity of a workpiece. In some cases a controlled lack of uniformity is desirable. This would be difficult to achieve with the prior art arrangement, but can be provided by at least some embodiments of the invention.
From one aspect the present invention consists in plasma processing apparatus including a processing chamber having a working volume of regular cross-section such that an axis passes through the centre of symmetry of a pair of opposed faces of the volume, and a plurality of separate Radio Frequency (RF) plasma generating antennae for predominately inducing an electric field, each having a coil portion of geometric shape corresponding to the shape of the cross-section, disposed at or adjacent to a wall of the chamber with the portions being coaxial with each other and set at different distances from said axis.
The matching of the geometry of the coils with the cross-section of the working volume of the chamber means that the plasma which is struck conforms essentially to the chamber. Further the provision of these independent coils in this array allows independent control of the coils and hence control of the plasma between different parts of the working volume. Thus, in a preferred embodiment, the apparatus further comprises means for varying the effective output of the antennae. For example the means may vary the magnitude, frequency and/or relative phase of the RF power supplied and/or the relative spacing between the coils and the working volume.
The cross-section, and hence the coil portions, may be circular, square, rectangular, hexagonal or octagonal. The coil portions may surround the chamber, but it is preferred that they are disposed adjacent to or on the end of the chamber. The chamber may include at least one dielectric window adjacent to a coil portion. This window may be re-entrant relative to the chamber and there may be a plurality of discrete windows each extending parallel to and over at least the width of an associated coil portion. Thus, when the coil portions are generally circular, there may be a series of spaced annular windows.
The apparatus may further include at least one detector for detecting the degree of uniformity of the plasma or of a plasma driven process and feedback means for varying the effective relative outputs of the antennae to improve the uniformity.
From another aspect the invention consists in plasma processing apparatus including a processing chamber having a working volume, at least one Radio-Frequency (RF) plasma-generating antenna, the or each antenna being disposed in a respective dielectric trough located in a wall of the chamber.
The or each antenna may be non-planar and may transfer power through the inner and outer wall and the base of the trough, in which case one turn may lie adjacent one wall of the trough, another adjacent the other wall and a third adjacent the base.
From a still further aspect the invention consists in plasma processing apparatus wherein the antennae and an associated power source are designed to create a plasma of a predetermined skin depth and the antennae are separated by at least twice said predetermined skin depth.
The invention further includes plasma processing apparatus including a processing chamber having a working volume, a plurality of Radio-Frequency (RF) plasma generating antennae and an associated power source designed to create a plasma of a predetermined skin depth, and said antennae being separated by at least twice said predetermined skin depth.
Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above in the following description.